Intrinsically conductive polymers possess conjugated π-electron systems along their backbone, giving them the ability to support positive and negative charge carriers with controllable mobility along the chains [see Friend, R. H., “Conductive Polymers from Science to Applications” University of Cambridge, 1993]. In addition to the metallic and semiconductor characteristics, they also exhibit unique electroluminescence properties. Thus, a variety of opto-electronic applications including electromagnetic shield, electronic wire, anti-corrosion layer, transistor, sensor, solar cell, memory storage and light emitting diode have been explored. Since the first discovery approximately 25 years ago [Chiang, C. K.; Fincher, C. R., Jr.; Park, Y. W; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G., Physical Review Letters. 1977, 39, 1098], the conductive polymers have attracted strong interests in both industrial and academic areas. However, only rather limited commercial products have been developed so far. Among the various reasons for the limited commercial products and applications, purification of these polymers remains to be one major technical challenge.
Even a small amount of impurities present in the conductive polymers may act as charge trappers or photo quenchers, and alter their semiconductor characteristics. These impurities may be metallic and non-metallic, in neutral or ionic forms. For instance, in light emitting devices fabricated from conductive polymers, metallic impurities may act as recombination centers for injected charge carriers, leading to an increase in the recombination rates. The increased recombination rates often degrade the light emitting efficiency and increase the leakage current of the devices. If the metal content in a given electronic polymer system is not controllable, its semi-conductive behavior will not be predictable.
However, during the synthesis of the conjugated polymers, a relatively large amount of certain transition metallic-based materials is often used as catalysts [T. Yamamoto, Synlett, 2003, No.4, 425]. For the well-known conjugated polymeric system, polyfluorenes, the synthesis example was first given by Pei and Yang [Pei, Q. B., Yang, Y., Journal of American Chemical Society, 1996, 118, 7416]. In the Pei and Yang synthesis method, the polymerization of 2,7-dibromo-9,9-diakylfluorenes with NiII salt/zinc was specifically described. Hence, it is anticipated that a certain quantity of Ni and other metallic species will remain in the final polymer products. In U.S. Pat. No. 5,777,070 entitled ‘Process for preparing conjugated polymers’ issued on Jul. 7, 1998 to Inbasekaran, M., Wu, W. and Edmund, P. W., a synthesis method involving the Suzuki-type cross-coupling catalyzed by active palladium has been described. In this method, it is anticipated that certain amount of palladium and certain other metallic species will be present in the final polymers. More recently, Yamamoto-type coupling has been shown to lead to a higher molecular weight polymer for aryl-aryl coupling from dihaloaryls, in which more than two equivalent Ni(COD) (bis(1,5)cyclooctadiene) nickel(0)) are added as catalyst [Scherf, U., List, E. J. W., Advanced Materials, 2002, 14(7), 477]. In this last reported method, a certain amount of Ni is anticipated with other impurities in the final polymer products. From the above description, it is clear that metallic components are needed as catalysts in order to synthesize the conductive polymers, and the amount of these catalysts used varies from one reaction to the other. Moreover, the catalyst residue left inside the resulted polymer varies, depending on the reaction and the treatment procedures after synthesis. These metallic components may often remain in the conductive polymers after the synthesis and constitute impurities to affect the opto-electronic properties. Apart from these catalyst residues, there are other sources of additional metallic impurities. The other impurity sources may be from monomers that are used for organometallic polycondensations [T. Yamamoto, Synlett, 2003, No.4, 425] and other reactants used during the synthesis. They also include various contaminants or even walls of metallic reactors and agitators, containing various types of metallic impurities. Hence, depending on polymer synthesis procedures and post-synthesis treatment, various metals such as Ni, Zn, Fe, Ag, Pt, Pd, Cd, Mg, etc might exist in the final polymer products. These metals can exist in different forms, ionic or neutral, bonded or even un-bonded.
Therefore, the control of metallic impurities in any polymers for any opto-electronic device fabrication is required in order to achieve good characteristics. Due the factors to be described below, the control of metallic impurities in the polymers represents a highly technical challenge.
Reports on the methods for metallic impurity removal from polymers in the literatures are relatively limited. In a typical synthesis process, washing the polymer with water after the synthesis is the simplest way to remove impurities [Inbasekaran, M., Wu, W. and Edmund, P. W., U.S. Pat. No. 5,777,070]. This is because most polymers, conductive polymers in particular, are not water soluble whereas metallic ions in salt forms and not trapped in the polymers are usually water soluble. Therefore, some of the metallic ions not trapped in the polymers can be removed by washing with pure water. However, for those metallic impurities trapped in polymeric matrix it is difficult to wash them away as the polymers are not soluble in water. Washing with a selected solvent via a Soxhlet is another commonly used method and has been described by Chen, T., Wu, X., and Rieke, R. [Journal of American Chemical Society, 1995, 117, 233]. In this method, a solvent with a very low solubility of the polymer to be purified is chosen as the agent to wash the polymer. This low solubility is selected in order to keep the polymer in a solid form and to remove the impurities. However, this method is not effective enough in removing metallic impurities particularly those trapped inside polymeric matrix. In yet another method, repeated precipitation of a polymer solution from a high solubility solvent (for instance, polyfluorene in chloroform) with a low solubility solvent like methanol or acetone was adopted in an attempt to remove impurities [Weinfurtner, K-H., Fujikawa, H., Tokito, S., Taga, Y., Applied Physics Letters, 2000, 76(18), 2502]. However, in this method, it is not clear how many impurities could be removed, especially these metallic species are not soluble in either organic solvent used. In addition, this technique could consume big amount of solvent. It is noted that if the metallic species have limited solubility in the low solubility solvent, these metallic species will still precipitate along with the polymer.